1. Field of the Invention
This invention relates to fuel cell systems employing fuel cell stacks having a pressure differential between the anode or fuel and cathode or oxidant sides of the stack and, in particular, to fuel cell systems of this type incorporating mechanisms for lessening this pressure differential.
2. Description of the Related Art
A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery, includes a negative or anode electrode and a positive or cathode electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gases are supplied. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate in between each cell.
In high temperature fuel cell stacks, it is desirable to minimize the pressure differential of the gases passing through the anode and cathode sides of the stacks. This is so because it is necessary in certain sections of the stack to provide seals to keep the fuel and oxidant gases isolated from each other. To create the required seals, surfaces, which is some cases sandwich a gasket, are mechanically forced together to realize an “acceptable” leak rate. This leak rate is a function of the pressure differential. Therefore, minimizing the pressure differential is important to prevent excessive leaks.
Keeping the pressure differential at a minimum has been achieved in past systems by attempting to cause the pressure of the oxidant gas at the inlet of the cathode-side of a stack to be equal to the pressure of the exhaust fuel gas at the exit of the anode-side of the stack. This has to be realized in the face of the other operating requirements which tend to make the pressures unequal.
In particular, in a fuel cell stack, fresh air usually serves as oxidant and is provided at the entry of the cathode-side of the stack. This fresh air is typically at ambient temperature and must be heated to the operating temperature of the stack. Heating of the air is conventionally accomplished by burning unused or exhaust fuel gas exiting from the anode-side of the stack in the incoming air. In terms of the process flow at the junction of the two streams, the gas pressure at the exit of the anode-side of the stack is coupled to the gas pressure at the inlet of the cathode-side of the stack. As such, the pressure at the exit of the anode-side is necessarily higher than the pressure at the inlet of the cathode-side by the amount required to overcome the pressure losses associated with any connection piping and with the oxidizer used to burn the gases.
Current fuel cell systems have used a variety of approaches in solving this differential pressure problem. One such approach, utilizes a high temperature booster blower placed between the exit of the anode-side of the stack and the mixing point to overcome the pressure loss of the connection piping and oxidizer. This has the advantage of independently controlling the pressure balance but adds significant cost and reliability issues to a commercial system. Another approach, uses a downstream, hot recycle blower to draw both the anode exhaust gas and fresh air oxidant gas through a mixing device and oxidizer. This system configuration allows the gas pressure at the inlet on the cathode-side to run higher than the gas pressure at the exit on the anode-side with some control over the difference. Disadvantages to this system are, again, the cost and reliability of the recycle blower as well as the overall complexity of the system hardware. A further approach to the problem is to simply allow the fuel pressure to run higher than the oxidant pressure. The experience in this case is that a multitude of operating problems can arise. Problems include non-uniform stack temperatures, reduced system efficiency and elevated exhaust pollutant emissions.
It is, therefore, an object of the present invention to provide a fuel cell system employing a fuel cell stack in which the differential pressure between the gas at the inlet of the cathode-side and the gas at the exit of the anode-side of the stack is reduced in a manner which avoids the above disadvantages.
It is a further object of the present invention to provide a fuel cell system employing a fuel cell stack in which the differential pressure between the gas at the inlet of the cathode-side and the gas at the exit of the anode-side of the stack is reduced in a simple and easy manner.